feat: Support for environment variables substitution in config file

[dadrus]

This PR closes #67

Example configuration which shows the new possibility.

id: hydra_authenticator
type: oauth2_introspection
config:
  introspection_endpoint:
    url: http://hydra:4445/oauth2/introspect
    auth:
      type: basic_auth
      config:
        user: ${INTROSPECT_EP_USER}
        password: ${INTROSPECT_EP_PASSWORD}

This PR is a replacement for #378, which tried to implement the same functionality, unfortunately using a library, which clashed with go templates. Template support is now covered by the tests as well

[codecov]

Codecov Report

Merging #381 (25de2b3) into main (fa6f965) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
+ Coverage   86.43%   86.50%   +0.06%     
==========================================
  Files         193      193              
  Lines        7743     7753      +10     
==========================================
+ Hits         6693     6707      +14     
+ Misses        898      894       -4     
  Partials      152      152              

Impacted Files	Coverage Δ
internal/config/parser/yaml.go	100.00% <100.00%> (ø)
internal/rules/repository.go	97.29% <0.00%> (+3.60%)	⬆️

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